Notch is a type I transmembrane protein present on a cell surface. It contains a repeated EGF-like domain in its extracellular domain and NICD (Notch Intracellular Cytoplasmic Domain), which is a transcription factor containing an ankyrin repeated domain, in its intracellular domain. It has been known that Notch plays a role in intracellular signaling relating to cell differentiation. For example, in the developmental process of a cranial nerve system, some of the cells derived from ectoderm differentiate into neuronal precursor cells (stem cells) and further into nerve cells or glial cells, during which intracellular signaling via Notch is important. The mechanism of the intracellular signaling via Notch is as follows. First, Notch is expressed as a receptor on a Notch signal-receiving cell. During the transport to the cell surface, the Notch undergoes the cleavage at the extracellular domain (S1) by a protease such as furin, and the two Notch fragments resulting from the S1 cleavage are held together through an S-S bond on the cell surface. Next, when a Notch signal-sending cell is present near the Notch signal-receiving cell, a Notch ligand (e.g., Delta, Serrate, or Lag-2, belonging to a DSL family) is expressed on the surface of the Notch signal-sending cell. Under these two conditions, the Notch ligand interacts with the Notch receptor on the cell surface, whereby sequential proteolytic events are induced to trigger signal transduction. More specifically, the Notch is cleaved at a site (S2) close to the cell surface, which triggers the cleavage at a site (S3) that is either inside the cell membrane or in close proximity to the cell membrane inside the cell. NICD, which is the intracellular domain of the Notch resulting from the S3 cleavage, is released to an intracellular space and translocates to the nucleus, where it binds to a CSL family (CPB, SuH, or Lag-1; transcription factor) to regulate the transcription of target genes. Presenilin, which is associated with Alzheimer's disease, is involved in the S3 cleavage.
As described above, Notch plays an extremely important role in intracellular signaling for cell differentiation. Moreover, recent studies have revealed that Notch is involved not only in the differentiation of a cranial nerve system as described above but also in cell tumorigenesis, apoptosis, Alzheimer's disease, etc., which causes Notch to become a focus of attention (see Okochi et al., “Biology of Alzheimer's disease and presenilin”, Bunshi Seishin Igaku, Vol. 1, No. 3, 2001; Kageyama et al., “Notch pathway in neural development”, Tanpakushitsu Kakusan Koso, Vol. 45, No. 3, 2000; and Brian et al., “A carboxy-terminal deletion mutant of Notch 1 accelerates lymphoid oncogenesis in E2A-PBX1 transgenic mice”, Blood, Vol. 96, No. 5, Sep. 1, 2000, pp 1906-1913). Therefore, the detection of Notch signal transduction is extremely important for research and diagnosis of cell differentiation, cell tumorigensis, apoptosis, Alzheimer's disease, etc., and the earlier possible establishment of the technology for detecting Notch signal transduction is being demanded.